Not applicable.
The invention concerns a method for regulation of the initial part of the dryer section in a paper machine, in which method moisture is removed from the paper web in the dryer section and in which method the paper web is dried in the dryer section, in which method the paper web to be dried is passed from the press section into the dryer section into the first group of drying cylinders, in which the paper web is dried against heated faces of drying cylinders.
Further, the invention concerns an equipment for regulation of the initial part of the dryer section in a paper machine, which equipment has been fitted in connection with the press section and with the dryer section in the paper machine.
As is known from the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw the groups of drying cylinders comprise two wires, which press the web, one from above and the other one from below, against heated cylinder faces. In twin-wire draw, between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular in the stages of drying in which the web is still relatively moist and, therefore, of low strength. This is why, in the course of the last 15 years, ever increasing use has been made of the single-wire draw, in which each group of drying cylinders comprises just one drying wire, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against heated cylinder faces, and on the reversing cylinders or rolls placed between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls inside the loop. In order to heat the faces of the drying cylinders, steam is passed into their interior, and the temperature of the surface of a heated drying cylinder is regulated so that the pressure of the steam present in the interior of the cylinder is regulated.
In view of the cylinder drying and in view of the runnability of the paper web and of the quality properties of the paper, the first drying cylinders placed in the beginning of the dryer section are highly important. If the temperatures of the first drying cylinders are too high, the paper web has a tendency to follow the face of the cylinder, which causes problems of runnability and thereby restricts the running speed. Likewise, the brightness of the paper web becomes lower, the porosity increases, the roughness increases, formation of dust increases, and the bulk may become lower. On the other hand, if the temperatures of the first drying cylinders are too low, the drying cylinders in the initial part of the dryer section are inefficient.
During threading of the web, even though the steam pressure has normally been switched off for this time or the steam pressure has been lowered to a level lower than a normal running situation in respect of the cylinders in the initial part of the dryer section, most commonly by lowering the pressure in the principal steam group, the lead-in strip has adhered to the drying cylinders, because the faces of the first drying cylinders are hot since the energy-consuming paper web is not present on them, in which case, thus, the web does not consume the thermal energy of the drying cylinders. Adhering of the lead-in strip has caused threading problems and thereby lowered the efficiency of the paper machine.
In the prior-art multi-cylinder dryers, in the initial part of the dryer section, it has been a further problem that it has not been possible to use a temperature sufficiently high in view of optimal drying, because, when the paper web is in direct contact with the hot faces of the drying cylinders, at temperatures higher than a certain level the web tends to adhere to the hot faces of the cylinders, which results in web breaks and standstills. It has also been noticed that excessively hot contact drying cylinders have detrimental effects on the quality properties of the paper. On the other hand, excessively low temperatures result in lowered drying efficiency.
At the first drying cylinders, problems have also arisen from the fact that, when the paper web starts becoming warm, it stretches, and when it stretches, it loses some of its tension, in which connection the web tends to follow the drying cylinder, which may cause problems of runnability. Traditionally, these problems have been solved so that a difference in speed has been arranged between the press section and the dryer section in order to maintain the web tension. Further, differences in speed have been employed between the first dryer groups in order to produce an adequate tension. However, excessive differences of draw have negative effects on the properties of the final product, because, when the paper is pulled, in particular its strength properties both in the machine direction and in the cross direction change.
One prior-art solution of the type described above is known from the publication EP 0,769,587. In said prior-art arrangement the draw applied to the web is increased constantly so that a number of short dryer groups arc employed in the initial part of the dryer section, which is unfavourable in view of the costs, because, among other things, extra drives, wires and wire guides are needed.
Important factors which affect the temperatures of the first cylinders in a dryer section include, among other things, the paper grade to be produced, the web moisture after the press section, and the temperature of the paper web. Usually, information on these factors in combination with the steam pressures in the first drying cylinders employed in cylinder drying has been obtained empirically only.
The current development in paper manufacture is about to lead in a situation in which there is no open draw between the press section and the dryer section, but a closed draw is concerned, in which case no high differences in speed can be employed, for which reason the problems described above in relation to the stretch of the web are emphasized.
The object of the present invention is to provide a solution for the problems described above.
It is a further object of the invention to provide a drying method and a dryer section that makes use of said method in which adhering of the web to the cylinders in the initial end of the dryer section is prevented and in which, at the same time, improved quality of the paper and improved runnability of the paper machine are achieved.
In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in that, in the method, the surface temperatures of the first drying cylinders in the dryer section are measured, that, in the method, the dry solids content/moisture content of the paper web and the temperature of the paper web before said first drying cylinders are measured, and that, in the method, in view of producing the desired running situation, based on the measurement results obtained, the surface temperatures of said first drying cylinders and/or the dry solids content/moisture content and/or the temperature of the paper web before said first drying cylinders is/are regulated.
On the other hand, the dryer section in accordance with the invention is mainly characterized in that the equipment comprises measurement devices fitted in connection with the first drying cylinders in the dryer section for measurement of the surface temperatures of said first drying cylinders, a measurement device for measurement of the dry solids content/moisture content of the paper web and of the temperature of the paper web before said first drying cylinders, and a control device for regulation of the surface temperatures of said first drying cylinders and/or of the dry solids content/moisture content and/or of the temperature of the paper web before said first drying cylinders, based on the measurement results obtained by means of the measurement devices, to produce the desired running situation.
In accordance with the invention, the surface temperatures of the first drying cylinders in the dryer section and the dry solids/moisture content of the paper web and the temperature of the web before the first drying cylinders are measured, and on the basis of the measurement results the surface temperatures of the first drying cylinders are regulated to the desired level so that a good running situation is achieved.
Besides by means of the steam pressure/flow, the surface temperatures of the first drying cylinders can, of course, also be regulated by means of other known methods, for example by means of induction heating or by means of electrical resistors.
The highest possible difference in temperature that provides good runnability depends on a number of different factors: paper grade and basis weight, stock that is used, web temperature, moisture content, fiber orientation, and machine speed. This is why it is often not possible to present a precise model for a correct maximal difference in temperature, but the regulation algorithms must be based on empiric information. It is known that the strength properties of different paper grades depend on the fibrous raw-material that is used and on the various properties of said material. The fibrous raw-material, its grinding degree, and possible additives and fillers affect the tendency with which the web, when moist, attempts to adhere to a hot cylinder face. In practice, these are, however, factors which cannot be changed for the sake of the runnability in the initial part of the dryer section alone, but the decisive factor is the paper quality. The same is true concerning the fiber orientation, even though it is known that a highly oriented web would be stronger in the machine direction.
The web temperature has two factors which act in opposite directions. On one hand, it has been noticed that, when the web is hotter, it does not adhere to the first hot faces equally well. On the other hand, the web strength becomes lower when the temperature becomes higher. The machine speed does not have a direct intensive effect on the adhering of the web to the first cylinder, but, on the other hand, it produces a vacuum in the opening web-cylinder gap, as a result of which vacuum the web tends to follow the cylinder face. This diverts the web from the face of the support wire and, thus, subjects the web to centrifugal forces which are increased in direct proportion to the second power of the speed.
The web moisture has a great negative effect on the runnability of the initial part of the dryer section. First, experience has shown that a moist web has an intensive tendency to adhere to a cylinder face. In practice, this has caused, e.g., formation of dust and a so-called linting effect. On the other hand, the web strength becomes lower when the moisture content becomes higher.
A solution is included in the scope of the present invention in which a regulation model has been formed by using at least some of the parameters mentioned above and by regulating the surface temperatures of the first drying cylinders so that the difference between the temperatures of the cylinders and the web is smaller than the maximal temperature given by said model. In its simplest form, this model can have, for example, the following form:
xcex94Y=T0xe2x88x92K*M, wherein
T0 is an empiric invariable or an invariable determined by computing,
K is an empiric invariable or an invariable determined by computing, and
M is the moisture content of the web before the first cylinder.
One such equation of regulation might be
xe2x80x83xcex94T(xc2x0 C.)=48xe2x88x925*M, wherein
M is the moisture content of the web before the first cylinder as a percentage of the total weight of the web.
The equation is valid in the range 35% less than M less than 60%
A form that is slightly more common is represented by
xcex94T=T0(Tr, BW)xe2x88x92k1*f(M)xe2x88x92k2*f(v), wherein
T0 is an invariable dependent on web temperature Tr and on basis weight BW,
k1 and k2 are invariable coefficients, and
(M) and f(v) are functions of web moisture M and on machine speed v.
An example of such a regulation model is the equation:
xcex94T(xc2x0 C.)=60xe2x88x926*Mxe2x88x920.01*v2, wherein
v is the machine speed (meters per second).
Of course, in its commonest form, the regulation model is, for each paper grade and paper quality to be produced, of the form:
xcex94T=xcex94T(Tr, v, M, BW),
but, as was stated earlier, this is often quite difficult to determine precisely.
The control algorithm solutions described above are also affected by limitations arising from quality factors, so that the initial part of the dryer section is quite significant from the point of view of the paper quality. In the initial part of the dryer section the web may end up in such a range of temperature-moisture that the fibers are softened. When the initial drying temperature becomes higher, among other things the following changes are noticed in the properties of the web: the coefficient of scattering of light becomes lower, the tensile strength and density become higher, the permeability to air is lowered, and the roughness of the surface is increased. At the same time the calendering quality of the paper is deteriorated. The magnitude of the effect depends on the yield of stock so that the effects are highest with mechanical pulps, but are reduced rapidly when the yield becomes lower. One consequence of the softening of the fibers by the effect of the drying temperature is adhering of fibers to the faces of the cylinders in the initial part of the dryer section if the temperatures of the cylinders are excessively high in the beginning, which effect occurs with printing papers that contain mechanical pulp. When the fibers adhere to the cylinders, they rise upright when the web is separated from the cylinder. This causes dust formation in the paper during printing. A certain contribution to such adhesion is given by various additives of paper, such as stock sizes. In accordance with the present invention, these factors present in the initial part of the dryer section are taken into account when the surface temperatures of the drying cylinders are regulated in accordance with the invention so that a good running situation is achieved.
The surface temperatures of the drying cylinders can be measured by means of IR measurements, for example by means of an IR camera, or by using particular apparatuses developed for measurement of the surface temperature of a cylinder. Such an apparatus has been described, for example in the journal Paper Technology, June 1998, page 17. From the prior art, various temperature detectors that reach contact with the face of a drying cylinder are also known.
After a paper web break, the steam pressures in the first cylinders are lowered from the level of a normal running situation. Based on the results obtained from measurements of the surface temperatures of the first drying cylinders, the surface temperatures of the first drying cylinders are regulated to a suitable level for the time of threading, and after threading the temperatures are raised, based on a pre-determined sequence or on measurement of the ultimate dry solids content, to the level corresponding to the normal running situation. If necessary, the moisture content and the temperature of the lead-in strip before the first drying cylinders can be measured.
Based on the moisture content of the lead-in strip, the possibilities of success of the following threading can be estimated, because the moisture value predicts the runnability. An excessively moist lead-in strip is of low strength, and it adheres readily to the first drying cylinders. It also stretches easily and makes successful threading of the web more difficult.
Based on the measurement results, if necessary, it is also possible to regulate the press loads in the press section in order to obtain sufficient strength of the lead-in strip so as to provide a successful threading. In addition to this, based on the measurement results, it is possible to select suitable differences of draw between the press section and the dryer section and between the first groups in the dryer section as a function of the speed.
In accordance with a favourable additional feature of the invention, in the event of a web break, the steam pressures in the first cylinders are lowered in the usual way, and after this the evaporation load is equalized by moistening the wire. The web moisture and the web temperature preceding the first drying cylinders are measured after the web break in order that it should be possible to make the necessary corrections either by means of impingement drying, if there is an impingement drying unit in the initial part of the dryer section before the first drying cylinders, and/or by means of a steam box in the press section and/or by means of press loads. If the measurement also provides the cross-direction profiles of moisture and temperature, by means of said devices it is also possible to correct possible profile defects.
During normal running, based on the measurement results, it is possible to optimize the surface temperatures of the drying cylinders, the differences of draw in the initial part of the dryer section, and, if necessary, to change the press loads or to control the steam box of the press section so that an optimal running situation is achieved. Also, in a preferred embodiment of the present invention, based on the measurement results, it is possible to regulate the blow parameters of an impingement drying unit, which may have been fitted before the first drying cylinders, in compliance with the dry solids content and the temperature of the web.
In accordance with the invention, the surface temperatures of the first cylinders are monitored. The temperature is used as an indirect indicator of the conduct of the web on the cylinder concerned. Of course, it would also be possible to use, for example, apparatuses based on a video camera and picture processing, or a laser rangefinder, by whose means the conduct of the web in the opening wire-cylinder gap is monitored directly. Such a technology is, however, clearly more expensive and more complicated than measurement of temperature, and it is probable that even in such a case measurement of the surface temperature of the cylinder would be advantageous in view of regulation of the system.
In an additional embodiment of the invention, based on said measurements of the surface temperatures of the cylinders and/or on the measurements of the web moisture and/or web temperature, it is possible to regulate the vacuums at least in the first suction rolls and/or the capacities of the blow/suction boxes. With respect to the regulation of the vacuum, reference is made to U.S. Pat. No. 5,535,527. The applicant has noticed that, as a rule, when the web moisture and/or temperature become(s) higher, the web must be supported more efficiently in the initial part of the dryer section. The same is true regarding the surface temperature of a cylinder, but not to an equally great extent, in particular if the difference in temperature between the web and the cylinder is not excessively large.
In accordance with the present invention, the measurements can also be carried out with a web of full width in order that information could be obtained on the profile parameters of the web and that necessary regulation could be performed in respect of the profile properties of the web. In a preferred embodiment of the invention, the running parameters of the initial part of the dryer section are chosen on the basis of the most unfavourable point measured from the cross-direction profile.